In a wireless Positive Train Control (PTC) system, a fast-travelling locomotive communicates with a wayside or track-side base station through a radio link. A spectrum at 220 MHz has been allocated for the wireless PTC application, where orthogonal frequency division multiplexing (OFDM) is selected as the underlying modulation technology to provide a reliable communication link between a locomotive and base stations. It is well known for digital radio systems that there are various channel impairments due to noise, multipath propagation and the time-varying transmission media. Accordingly, channel equalization is often used to compensate channel impairments to improve system performance. For example, in U.S. Pat. Ser. No. 5/283,811, entitled “Decision Feedback Equalization for Digital Cellular Radio”, by Sandeep Chennakeshu et al., issued on Feb. 1, 1994, discloses an adaptive Decision Feedback Equalizer (DFE) for a digital cellular mobile radio channel demodulator employing a Complex Fast-Kalman Adaptation algorithm to track channel variations.
When a train travels at a high speed, the relative speed between the on-board radio terminal at the locomotive and the base-station radio terminal at wayside or track-side will cause a phenomenon of frequency shift in the received signal, called the Doppler Effect. The Doppler Effect becomes more prominent for signal at higher frequencies. For example, the wireless PTC system with a VHF signal at 220 MHz will be subject to a Doppler frequency shift of 102.1 Hz when the locomotive travels at a speed of 500 km/h (138.89 m/s) relative to the base station. In a real environment, the signal received from the transmitter often travels through multiple paths to arrive at the receiver. The overall result of Doppler Effect and multi-path reception will cause fluctuations in the received signal associated with the Doppler frequency.
In practice, the base station is located in the proximity of the railroad track. When the locomotive passes by the base station, the Doppler frequency will rapidly change from a positive frequency offset to zero, and to a negative frequency offset. In the above example, the Doppler frequency will change from +101.8 Hz to −101.8 Hz in a very short period of time. If the system design does not take into account of fast changing Doppler frequency near the base station, the system may fail, such as loss of phase tracking. For example, if a slow frequency tracking loop is used, the frequency may have changed substantially before the tracking loop converges. On the other hand, if a fast tracking loop is always used, the performance of the equalizer may be compromised. Consequently, it is desirable to design a system that can adaptively select proper tracking speed according to variation of the channel condition.